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<p>A function template defines a family of functions.
</p>
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<div id="toctitle"><h2>Contents</h2></div>
<ul>
<li class="toclevel-1 tocsection-1"><a href="function_template.html%23Syntax"><span class="tocnumber">1</span> <span class="toctext">Syntax</span></a></li>
<li class="toclevel-1 tocsection-2"><a href="function_template.html%23Explanation"><span class="tocnumber">2</span> <span class="toctext">Explanation</span></a></li>
<li class="toclevel-1 tocsection-3">
<a href="function_template.html%23Function_template_instantiation"><span class="tocnumber">3</span> <span class="toctext">Function template instantiation</span></a>
<ul>
<li class="toclevel-2 tocsection-4"><a href="function_template.html%23Explicit_instantiation"><span class="tocnumber">3.1</span> <span class="toctext">Explicit instantiation</span></a></li>
<li class="toclevel-2 tocsection-5"><a href="function_template.html%23Implicit_instantiation"><span class="tocnumber">3.2</span> <span class="toctext">Implicit instantiation</span></a></li>
</ul>
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<li class="toclevel-1 tocsection-6"><a href="function_template.html%23Template_argument_deduction"><span class="tocnumber">4</span> <span class="toctext">Template argument deduction</span></a></li>
<li class="toclevel-1 tocsection-7"><a href="function_template.html%23Explicit_template_arguments"><span class="tocnumber">5</span> <span class="toctext">Explicit template arguments</span></a></li>
<li class="toclevel-1 tocsection-8"><a href="function_template.html%23Template_argument_substitution"><span class="tocnumber">6</span> <span class="toctext">Template argument substitution</span></a></li>
<li class="toclevel-1 tocsection-9"><a href="function_template.html%23Function_template_overloading"><span class="tocnumber">7</span> <span class="toctext">Function template overloading</span></a></li>
<li class="toclevel-1 tocsection-10"><a href="function_template.html%23Function_template_specialization"><span class="tocnumber">8</span> <span class="toctext">Function template specialization</span></a></li>
<li class="toclevel-1 tocsection-11"><a href="function_template.html%23See_Also"><span class="tocnumber">9</span> <span class="toctext">See Also</span></a></li>
</ul>
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<h3>
<span class="editsection">[<a href="http%3A//en.cppreference.com/mwiki/index.php" title="Edit section: Syntax">edit</a>]</span> <span class="mw-headline" id="Syntax">Syntax</span>
</h3>
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<td class="t-sdsc-nopad"> <code><b>template</b></code> <code><b>&lt;</b></code> <span class="t-spar">parameter-list</span> <code><b>&gt;</b></code> <span class="t-spar">declaration</span>
</td>
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<h3>
<span class="editsection">[<a href="http%3A//en.cppreference.com/mwiki/index.php" title="Edit section: Explanation">edit</a>]</span> <span class="mw-headline" id="Explanation">Explanation</span>
</h3>
<p><span class="t-spar">declaration</span> defines or declares a <a href="class_template.html" title="cpp/language/class template">class (including struct and union), a member class or member enumeration type</a>, a function or member function, a static data member of a class template, or a <a href="type_alias.html" title="cpp/language/type alias">type alias</a>. It may also define a <a href="template_specialization.html" title="cpp/language/template specialization">template specialization</a>. This page focuses on function templates.
</p>
<p><span class="t-spar">parameter-list</span> is a non-empty comma-separated list of the template parameters, each of which is either non-type parameter, a type parameter, a template parameter, or a <a href="parameter_pack.html" title="cpp/language/parameter pack">parameter pack</a> of any of those. For function templates, template parameters are declared in the same manner as for class templates: see <a href="class_template.html" title="cpp/language/class template">class template</a> page for details.
</p>
<h3>
<span class="editsection">[<a href="http%3A//en.cppreference.com/mwiki/index.php" title="Edit section: Function template instantiation">edit</a>]</span> <span class="mw-headline" id="Function_template_instantiation">Function template instantiation</span>
</h3>
<p>A function template by itself is not a type, or a function, or any other entity. No code is generated from a source file that contains only template definitions. In order for any code to appear, a template must be instantiated: the template arguments must be determined so that the compiler can generate an actual function (or class, from a class template).
</p>
<h4>
<span class="editsection">[<a href="http%3A//en.cppreference.com/mwiki/index.php" title="Edit section: Explicit instantiation">edit</a>]</span> <span class="mw-headline" id="Explicit_instantiation">Explicit instantiation</span>
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<td> <code><b>template</b></code> <span class="t-spar">return-type</span> <span class="t-spar">name</span> <code><b>&lt;</b></code>  <span class="t-spar">argument-list</span> <code><b>&gt;</b></code> <code><b>(</b></code> <span class="t-spar">parameter-list</span> <code><b>)</b></code> <code><b>;</b></code>
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<td> (1)
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<td> <code><b>template</b></code> <span class="t-spar">return-type</span> <span class="t-spar">name</span> <code><b>(</b></code> <span class="t-spar">parameter-list</span> <code><b>)</b></code> <code><b>;</b></code>
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<td> (2)
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<td> <code><b>extern</b></code> <code><b>template</b></code> <span class="t-spar">return-type</span> <span class="t-spar">name</span> <code><b>&lt;</b></code> <span class="t-spar">argument-list</span> <code><b>&gt;</b></code> <code><b>(</b></code> <span class="t-spar">parameter-list</span> <code><b>)</b></code> <code><b>;</b></code>
</td>
<td> (3)
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<td> <span class="t-mark-rev t-since-cxx11">(since C++11)</span>
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<td> <code><b>extern</b></code> <code><b>template</b></code> <span class="t-spar">return-type</span> <span class="t-spar">name</span> <code><b>(</b></code> <span class="t-spar">parameter-list</span> <code><b>)</b></code> <code><b>;</b></code>
</td>
<td> (4)
</td>
<td> <span class="t-mark-rev t-since-cxx11">(since C++11)</span>
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<div class="t-li1">
<span class="t-li">1)</span> Explicit instantiation definition (without <a href="template_argument_deduction.html" title="cpp/language/template argument deduction">template argument deduction</a> if every non-default template parameter is explicitly specified)</div>
<div class="t-li1">
<span class="t-li">2)</span> Explicit instantiation definition with template argument deduction for all parameters</div>
<div class="t-li1">
<span class="t-li">3)</span> Explicit instantiation declaration (without template argument deduction if every non-default template parameter is explicitly specified)</div>
<div class="t-li1">
<span class="t-li">4)</span> Explicit instantiation declaration with template argument deduction for all parameters</div>
<p>An explicit instantiation definition forces instantiation of the function or member function they refer to. It may appear in the program anywhere after the template definition, and for a given argument-list, is only allowed to appear once in the program.
</p>
<p>An explicit instantiation declaration (an extern template) prevents implicit instantiations: the code that would otherwise cause an implicit instantiation has to use the explicit instantiation definition provided somewhere else in the program.
</p>
<p><br>
A trailing template-argument can be left unspecified in an explicit instantiation of a function template specialization or of a member function template specialization if it can be <a href="template_argument_deduction.html" title="cpp/language/template argument deduction">deduced</a> from the function parameter
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">typename</span> T<span class="sy1">&gt;</span>
<span class="kw4">void</span> f<span class="br0">(</span>T s<span class="br0">)</span>
<span class="br0">{</span>
    <a href="../io/basic_ostream.html"><span class="kw1439">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> s <span class="sy1">&lt;&lt;</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
<span class="br0">}</span>
 
<span class="kw1">template</span> <span class="kw4">void</span> f<span class="sy1">&lt;</span><span class="kw4">double</span><span class="sy1">&gt;</span><span class="br0">(</span><span class="kw4">double</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// instantiates f&lt;double&gt;(double)</span>
<span class="kw1">template</span> <span class="kw4">void</span> f<span class="sy1">&lt;&gt;</span><span class="br0">(</span><span class="kw4">char</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// instantiates f&lt;char&gt;(char), template argument deduced</span>
<span class="kw1">template</span> <span class="kw4">void</span> f<span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// instantiates f&lt;int&gt;(int), template argument deduced</span></pre></div></div>
<p>Explicit instantiation of a function template or of a member function of a class template cannot use <code>inline</code> or <code>constexpr</code>. If the declaration of the explicit instantiation names an implicitly-declared special member function, the program is ill-formed.
</p>
<p>Explicit instantiation declarations do not suppress the implicit instantiation of <a href="inline.html" title="cpp/language/inline">inline</a> functions, <a href="auto.html" title="cpp/language/auto">auto</a>-declarations, references, and class template specializations. (thus, when the inline function that is a subject of explicit instantiation declaration is ODR-used, it is implicitly instantiated for inlining, but its out-of-line copy is not generated in this TU)
</p>
<p>Explicit instantiation definition of a function template with <a href="default_arguments.html" title="cpp/language/default arguments">default arguments</a> is not a use of the arguments, and does not attempt to initialize them:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw4">char</span><span class="sy2">*</span> p <span class="sy1">=</span> <span class="nu0">0</span><span class="sy4">;</span>
<span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> T<span class="sy1">&gt;</span> T g<span class="br0">(</span>T x <span class="sy1">=</span> <span class="sy3">&amp;</span>p<span class="br0">)</span> <span class="br0">{</span> <span class="kw1">return</span> x<span class="sy4">;</span> <span class="br0">}</span>
<span class="kw1">template</span> <span class="kw4">int</span> g<span class="sy1">&lt;</span><span class="kw4">int</span><span class="sy1">&gt;</span><span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span><span class="sy4">;</span>   <span class="co1">// OK even though &amp;p isn’t an int.</span></pre></div></div>
<h4>
<span class="editsection">[<a href="http%3A//en.cppreference.com/mwiki/index.php" title="Edit section: Implicit instantiation">edit</a>]</span> <span class="mw-headline" id="Implicit_instantiation">Implicit instantiation</span>
</h4>
<p>When code refers to a function in context that requires the function definition to exist, and this particular function has not been explicitly instantiated, implicit instantiation occurs. The list of template arguments does not have to be supplied if it can be <a href="template_argument_deduction.html" title="cpp/language/template argument deduction">deduced</a> from context
</p>
<div class="t-example">
<div class="t-example-live-link"><div class="coliru-btn coliru-btn-run-init">Run this code</div></div>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="co2">#include &lt;iostream&gt;</span>
 
<span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">typename</span> T<span class="sy1">&gt;</span>
<span class="kw4">void</span> f<span class="br0">(</span>T s<span class="br0">)</span>
<span class="br0">{</span>
    <a href="../io/basic_ostream.html"><span class="kw1439">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> s <span class="sy1">&lt;&lt;</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
<span class="br0">}</span>
 
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
    f<span class="sy1">&lt;</span><span class="kw4">double</span><span class="sy1">&gt;</span><span class="br0">(</span><span class="nu0">1</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// instantiates and calls f&lt;double&gt;(double)</span>
    f<span class="sy1">&lt;&gt;</span><span class="br0">(</span><span class="st0">'a'</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// instantiates and calls f&lt;char&gt;(char)</span>
    f<span class="br0">(</span><span class="nu0">7</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// instantiates and calls f&lt;int&gt;(int)</span>
    <span class="kw4">void</span> <span class="br0">(</span><span class="sy2">*</span>ptr<span class="br0">)</span><span class="br0">(</span><a href="../string/basic_string.html"><span class="kw1045">std::<span class="me2">string</span></span></a><span class="br0">)</span> <span class="sy1">=</span> f<span class="sy4">;</span> <span class="co1">// instantiates f&lt;string&gt;(string)</span>
<span class="br0">}</span></pre></div></div>
<p><br> 
</p>
</div>
<p>Note: omitting <code>&lt;&gt;</code> entirely allows <a href="overload_resolution.html" title="cpp/language/overload resolution">overload resolution</a> to examine both template and non-template overloads.
</p>
<h3>
<span class="editsection">[<a href="http%3A//en.cppreference.com/mwiki/index.php" title="Edit section: Template argument deduction">edit</a>]</span> <span class="mw-headline" id="Template_argument_deduction">Template argument deduction</span>
</h3>
<p>In order to instantiate a <strong class="selflink">function template</strong>, every template argument must be known, but not every template argument has to be specified. When possible, the compiler will deduce the missing template arguments from the function arguments. This occurs when a function call is attempted and when an address of a function template is taken.
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">typename</span> To, <span class="kw1">typename</span> From<span class="sy1">&gt;</span> To convert<span class="br0">(</span>From f<span class="br0">)</span><span class="sy4">;</span>
 
<span class="kw4">void</span> g<span class="br0">(</span><span class="kw4">double</span> d<span class="br0">)</span> 
<span class="br0">{</span>
    <span class="kw4">int</span> i <span class="sy1">=</span> convert<span class="sy1">&lt;</span><span class="kw4">int</span><span class="sy1">&gt;</span><span class="br0">(</span>d<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// calls convert&lt;int,double&gt;(double)</span>
    <span class="kw4">char</span> c <span class="sy1">=</span> convert<span class="sy1">&lt;</span><span class="kw4">char</span><span class="sy1">&gt;</span><span class="br0">(</span>d<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// calls convert&lt;char,double&gt;(double)</span>
    <span class="kw4">int</span><span class="br0">(</span><span class="sy2">*</span>ptr<span class="br0">)</span><span class="br0">(</span><span class="kw4">float</span><span class="br0">)</span> <span class="sy1">=</span> convert<span class="sy4">;</span> <span class="co1">// instantiates convert&lt;int, float&gt;(float)</span>
<span class="br0">}</span></pre></div></div>
<p>This mechanism makes it possible to use template operators, since there is no syntax to specify template arguments for an operator other than by re-writing it as a function call expression.
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="co2">#include &lt;iostream&gt;</span>
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span> 
<span class="br0">{</span>
    <a href="../io/basic_ostream.html"><span class="kw1439">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">"Hello, world"</span> <span class="sy1">&lt;&lt;</span> <a href="../io/manip/endl.html"><span class="kw1472">std::<span class="me2">endl</span></span></a><span class="sy4">;</span>
    <span class="co1">// operator&lt;&lt; is looked up via ADL as std::operator&lt;&lt;,</span>
    <span class="co1">// then deduced to operator&lt;&lt;&lt;char, std::char_traits&lt;char&gt;&gt; both times</span>
    <span class="co1">// std::endl is deduced to &amp;std::endl&lt;char, std::char_traits&lt;char&gt;&gt;</span>
<span class="br0">}</span></pre></div></div>
<p>Template argument deduction takes place after the function template <a href="lookup.html" title="cpp/language/lookup">name lookup</a> (which may involve <a href="adl.html" title="cpp/language/adl">argument-dependent lookup</a>) and before <a href="overload_resolution.html" title="cpp/language/overload resolution">overload resolution</a>.
</p>
<p>See <a href="template_argument_deduction.html" title="cpp/language/template argument deduction">template argument deduction</a> for details.
</p>
<h3>
<span class="editsection">[<a href="http%3A//en.cppreference.com/mwiki/index.php" title="Edit section: Explicit template arguments">edit</a>]</span> <span class="mw-headline" id="Explicit_template_arguments">Explicit template arguments</span>
</h3>
<p>Template arguments of a function template may be obtained from
</p>
<ul>
<li> template argument deduction
</li>
<li> default template arguments
</li>
</ul>
<dl><dd>
<ul>
<li> specified explicitly in a function call expression
</li>
<li> specified explicitly when an address of a function is taken
</li>
<li> specified explicitly when a reference to function is initialized
</li>
<li> specified explicitly when a pointer to member function is formed
</li>
<li> specified in an explicit specialization
</li>
<li> specified in an explicit instantiation
</li>
<li> specified in a friend declaration
</li>
</ul>
</dd></dl>
<p>There is no way to explicitly specify template arguments to operators, conversion functions, and constructors, because they are called without the use of the function name.
</p>
<p>The specified template arguments must match the template parameters in kind (i.e., type for type, non-type for non-type, and template for template). There cannot be more arguments than there are parameters (unless one parameter is a parameter pack, in which case there has to be an argument for each non-pack parameter)
</p>
<p>The specified non-type arguments must either match the types of the corresponding non-type template parameters, or be <a href="template_parameters.html%23Template_non-type_arguments" title="cpp/language/template parameters">convertible to them</a>.
</p>
<p>The function parameters that do not participate in template argument deduction (e.g. if the corresponding template arguments are explicitly specified) are subject to implicit conversions to the type of the corresponding function parameter (as in the usual <a href="overload_resolution.html" title="cpp/language/overload resolution">overload resolution</a>).
</p>
<p>A template parameter pack that is explicitly specified may be extended by template argument deduction if there are additional arguments:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> ... <span class="me1">Types</span><span class="sy1">&gt;</span> <span class="kw4">void</span> f<span class="br0">(</span>Types ... <span class="me1">values</span><span class="br0">)</span><span class="sy4">;</span>
<span class="kw4">void</span> g<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span>
  f<span class="sy1">&lt;</span><span class="kw4">int</span><span class="sy2">*</span>, <span class="kw4">float</span><span class="sy2">*</span><span class="sy1">&gt;</span><span class="br0">(</span><span class="nu0">0</span>, <span class="nu0">0</span>, <span class="nu0">0</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// Types = {int*, float*, int}</span>
<span class="br0">}</span></pre></div></div>
<h3>
<span class="editsection">[<a href="http%3A//en.cppreference.com/mwiki/index.php" title="Edit section: Template argument substitution">edit</a>]</span> <span class="mw-headline" id="Template_argument_substitution">Template argument substitution</span>
</h3>
<p>When all template arguments have been specified, deduced or obtained from default template arguments, every uses of a template parameter in the function parameter list is replaced with the corresponding template arguments.
</p>
<p>Substitution failure (that is, failure to replace template parameters with the deduced or provided template arguments) of a function template removes the function template from the <a href="overload_resolution.html" title="cpp/language/overload resolution">overload set</a>. This allows a number of ways to manipulate overload sets using template metaprogramming: see <a href="sfinae.html" title="cpp/language/sfinae">SFINAE</a> for details.
</p>
<p>After substitution, all function parameters of array and function type are adjusted to pointers and all top-level cv-qualifiers are dropped from function parameters (as in a regular <a href="function.html%23Function_declaration" title="cpp/language/function">function declaration</a>).
</p>
<p>The removal of the top-level cv-qualifiers does not affect the type of the parameter as it appears within the function:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">template</span> <span class="sy1">&lt;</span><span class="kw1">class</span> T<span class="sy1">&gt;</span> <span class="kw4">void</span> f<span class="br0">(</span>T t<span class="br0">)</span><span class="sy4">;</span>
<span class="kw1">template</span> <span class="sy1">&lt;</span><span class="kw1">class</span> X<span class="sy1">&gt;</span> <span class="kw4">void</span> g<span class="br0">(</span><span class="kw4">const</span> X x<span class="br0">)</span><span class="sy4">;</span>
<span class="kw1">template</span> <span class="sy1">&lt;</span><span class="kw1">class</span> Z<span class="sy1">&gt;</span> <span class="kw4">void</span> h<span class="br0">(</span>Z z, Z<span class="sy2">*</span> zp<span class="br0">)</span><span class="sy4">;</span>
 
<span class="co1">// two different functions with the same type, but </span>
<span class="co1">// within the function, t has different cv qualifications</span>
f<span class="sy1">&lt;</span><span class="kw4">int</span><span class="sy1">&gt;</span><span class="br0">(</span><span class="nu0">1</span><span class="br0">)</span><span class="sy4">;</span>       <span class="co1">// function type is void(int), t is int</span>
f<span class="sy1">&lt;</span><span class="kw4">const</span> <span class="kw4">int</span><span class="sy1">&gt;</span><span class="br0">(</span><span class="nu0">1</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// function type is void(int), t is const const</span>
 
<span class="co1">// two different functions with the same type and the same x</span>
<span class="co1">// (pointers to these two functions are not equal,</span>
<span class="co1">//  and function-local statics would have different addresses)</span>
g<span class="sy1">&lt;</span><span class="kw4">int</span><span class="sy1">&gt;</span><span class="br0">(</span><span class="nu0">1</span><span class="br0">)</span><span class="sy4">;</span>       <span class="co1">// function type is void(int), x is const int</span>
g<span class="sy1">&lt;</span><span class="kw4">const</span> <span class="kw4">int</span><span class="sy1">&gt;</span><span class="br0">(</span><span class="nu0">1</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// function type is void(int), x is const int</span>
 
<span class="co1">// only top-level cv-qualifiers are dropped:</span>
h<span class="sy1">&lt;</span><span class="kw4">const</span> <span class="kw4">int</span><span class="sy1">&gt;</span><span class="br0">(</span><span class="nu0">1</span>, <a href="../types/NULL.html"><span class="kw103">NULL</span></a><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// function type is void(int, const int*) </span>
                       <span class="co1">// z is const int, zp is const int*</span></pre></div></div>
<h3>
<span class="editsection">[<a href="http%3A//en.cppreference.com/mwiki/index.php" title="Edit section: Function template overloading">edit</a>]</span> <span class="mw-headline" id="Function_template_overloading">Function template overloading</span>
</h3>
<p>More than one function templates and non-template functions may be overloaded.
</p>
<p>A non-template function is always distinct from a template specialization with the same type. Specializations of different function templates are always distinct from each other even if they have the same type. Two function templates with the same return type and the same parameter list are distinct and can be distinguished with explicit template argument list.
</p>
<p>When an expression that uses type or non-type template parameters appears in the function parameter list or in the return type, that expression remains a part of the function template signature for the purpose of overloading:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw4">int</span> I, <span class="kw4">int</span> J<span class="sy1">&gt;</span> A<span class="sy1">&lt;</span>I<span class="sy2">+</span>J<span class="sy1">&gt;</span> f<span class="br0">(</span>A<span class="sy1">&lt;</span>I<span class="sy1">&gt;</span>, A<span class="sy1">&lt;</span>J<span class="sy1">&gt;</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// overload #1</span>
<span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw4">int</span> K, <span class="kw4">int</span> L<span class="sy1">&gt;</span> A<span class="sy1">&lt;</span>K<span class="sy2">+</span>L<span class="sy1">&gt;</span> f<span class="br0">(</span>A<span class="sy1">&lt;</span>K<span class="sy1">&gt;</span>, A<span class="sy1">&lt;</span>L<span class="sy1">&gt;</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// same as #1</span>
<span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw4">int</span> I, <span class="kw4">int</span> J<span class="sy1">&gt;</span> A<span class="sy1">&lt;</span>I<span class="sy2">-</span>J<span class="sy1">&gt;</span> f<span class="br0">(</span>A<span class="sy1">&lt;</span>I<span class="sy1">&gt;</span>, A<span class="sy1">&lt;</span>J<span class="sy1">&gt;</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// overload #2</span></pre></div></div>
<p>Two expressions involving template parameters are called <i>equivalent</i> if two function definitions that contain these expressions would be the same under <a href="definition.html%23One_Definition_Rule" title="cpp/language/definition">ODR rules</a>, that is, the two expressions contain the same sequence of tokens whose names are resolved to same entities via name lookup, except template parameters may be differently named.
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">template</span> <span class="sy1">&lt;</span><span class="kw4">int</span> I, <span class="kw4">int</span> J<span class="sy1">&gt;</span> <span class="kw4">void</span> f<span class="br0">(</span>A<span class="sy1">&lt;</span>I<span class="sy2">+</span>J<span class="sy1">&gt;</span><span class="br0">)</span><span class="sy4">;</span>  <span class="co1">// template overload #1</span>
<span class="kw1">template</span> <span class="sy1">&lt;</span><span class="kw4">int</span> K, <span class="kw4">int</span> L<span class="sy1">&gt;</span> <span class="kw4">void</span> f<span class="br0">(</span>A<span class="sy1">&lt;</span>K<span class="sy2">+</span>L<span class="sy1">&gt;</span><span class="br0">)</span><span class="sy4">;</span>  <span class="co1">// equivalent to #1</span></pre></div></div>
 <table class="t-rev-begin">
<tr class="t-rev t-since-cxx14">
<td>
<p>When determining if two <a href="dependent_name.html" title="cpp/language/dependent name">dependent expressions</a> are equivalent, only the dependent names involved are considered, not the results of name lookup. If multiple declarations of the same template differ in the result of name lookup, the first such declaration is used:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">template</span> <span class="sy1">&lt;</span><span class="kw1">class</span> T<span class="sy1">&gt;</span> decltype<span class="br0">(</span>g<span class="br0">(</span>T<span class="br0">(</span><span class="br0">)</span><span class="br0">)</span><span class="br0">)</span> h<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// decltype(g(T())) is a dependent type</span>
<span class="kw4">int</span> g<span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span><span class="sy4">;</span>
<span class="kw1">template</span> <span class="sy1">&lt;</span><span class="kw1">class</span> T<span class="sy1">&gt;</span> decltype<span class="br0">(</span>g<span class="br0">(</span>T<span class="br0">(</span><span class="br0">)</span><span class="br0">)</span><span class="br0">)</span> h<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span> <span class="co1">// redeclaration of h() uses earlier lookup</span>
    <span class="kw1">return</span> g<span class="br0">(</span>T<span class="br0">(</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>                     <span class="co1">// ...although the lookup here does find g(int)</span>
<span class="br0">}</span>
<span class="kw4">int</span> i <span class="sy1">=</span> h<span class="sy1">&lt;</span><span class="kw4">int</span><span class="sy1">&gt;</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>   <span class="co1">// template argument substitution fails; g(int)</span>
                    <span class="co1">// was not in scope at the first declaration of h()</span></pre></div></div>
</td>
<td><span class="t-mark-rev t-since-cxx14">(since C++14)</span></td>
</tr>
</table>
<p>Two function templates are considered <i>equivalent</i> if
</p>
<ul>
<li> they are declared in the same scope
</li>
<li> they have the same name
</li>
<li> they have identical template parameter lists
</li>
<li> the expressions involving template parameters in their return types and parameter lists are <i>equivalent</i>
</li>
</ul>
<p>Two expressions involving template parameters are called <i>functionally equivalent</i> if they are not <i>equivalent</i>, but for any given set of template arguments, the evaluation of the two expressions results in the same value.
</p>
<p>Two function templates are considered <i>functionally equivalent</i> if they are <i>equivalent</i>, except that one or more expressions that involve template parameters in the 
</p>
<ul><li> they are <i>equivalent</i> (as above) except that one or more expressions that involve template  parameters in their return types and parameter lists are <i>functionally equivalent</i>
</li></ul>
<p>If a program contains declarations of function templates that are <i>functionally equivalent</i> but not <i>equivalent</i>, the program is ill-formed; no diagnostic is required.
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="co1">// equivalent</span>
<span class="kw1">template</span> <span class="sy1">&lt;</span><span class="kw4">int</span> I<span class="sy1">&gt;</span> <span class="kw4">void</span> f<span class="br0">(</span>A<span class="sy1">&lt;</span>I<span class="sy1">&gt;</span>, A<span class="sy1">&lt;</span>I<span class="sy2">+</span><span class="nu0">10</span><span class="sy1">&gt;</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// overload #1</span>
<span class="kw1">template</span> <span class="sy1">&lt;</span><span class="kw4">int</span> I<span class="sy1">&gt;</span> <span class="kw4">void</span> f<span class="br0">(</span>A<span class="sy1">&lt;</span>I<span class="sy1">&gt;</span>, A<span class="sy1">&lt;</span>I<span class="sy2">+</span><span class="nu0">10</span><span class="sy1">&gt;</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// redeclaration of overload #1</span>
 
<span class="co1">// not equivalent</span>
<span class="kw1">template</span> <span class="sy1">&lt;</span><span class="kw4">int</span> I<span class="sy1">&gt;</span> <span class="kw4">void</span> f<span class="br0">(</span>A<span class="sy1">&lt;</span>I<span class="sy1">&gt;</span>, A<span class="sy1">&lt;</span>I<span class="sy2">+</span><span class="nu0">10</span><span class="sy1">&gt;</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// overload #1</span>
<span class="kw1">template</span> <span class="sy1">&lt;</span><span class="kw4">int</span> I<span class="sy1">&gt;</span> <span class="kw4">void</span> f<span class="br0">(</span>A<span class="sy1">&lt;</span>I<span class="sy1">&gt;</span>, A<span class="sy1">&lt;</span>I<span class="sy2">+</span><span class="nu0">11</span><span class="sy1">&gt;</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// overload #2</span>
 
<span class="co1">// functionally-equivalent but not equivalent</span>
<span class="co1">// This program is ill-formed, no diagnostic required</span>
<span class="kw1">template</span> <span class="sy1">&lt;</span><span class="kw4">int</span> I<span class="sy1">&gt;</span> <span class="kw4">void</span> f<span class="br0">(</span>A<span class="sy1">&lt;</span>I<span class="sy1">&gt;</span>, A<span class="sy1">&lt;</span>I<span class="sy2">+</span><span class="nu0">10</span><span class="sy1">&gt;</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// overload #1</span>
<span class="kw1">template</span> <span class="sy1">&lt;</span><span class="kw4">int</span> I<span class="sy1">&gt;</span> <span class="kw4">void</span> f<span class="br0">(</span>A<span class="sy1">&lt;</span>I<span class="sy1">&gt;</span>, A<span class="sy1">&lt;</span>I<span class="sy2">+</span><span class="nu0">1</span><span class="sy2">+</span><span class="nu0">2</span><span class="sy2">+</span><span class="nu0">3</span><span class="sy2">+</span><span class="nu0">4</span><span class="sy1">&gt;</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// functionally equivalent</span></pre></div></div>
<p>When the same function template specialization matches more than one overloaded function template (this often results from <a href="template_argument_deduction.html" title="cpp/language/template argument deduction">template argument deduction</a>), <i>partial ordering of overloaded function templates</i> is performed to select the best match. 
</p>
<p>Specifically, partial ordering takes place in the following situations:
</p>
<div class="t-li1">
<span class="t-li">1)</span> overload resolution for a call to a function template specialization
<table class="metadata plainlinks ambox mbox-small-left ambox-notice" style=""><tr>
<td class="mbox-empty-cell"></td>
<td class="mbox-text" style="">This section is incomplete<br>Reason: mini-example </td>
</tr></table>
</div>
<div class="t-li1">
<span class="t-li">2)</span> when the address of a function template specialization is taken
<table class="metadata plainlinks ambox mbox-small-left ambox-notice" style=""><tr>
<td class="mbox-empty-cell"></td>
<td class="mbox-text" style="">This section is incomplete<br>Reason: mini-example </td>
</tr></table>
</div>
<div class="t-li1">
<span class="t-li">3)</span> when a <a href="../memory/new/operator_delete.html" title="cpp/memory/new/operator delete">placement operator delete</a> that is a function template specialization is selected to match a placement operator new
<table class="metadata plainlinks ambox mbox-small-left ambox-notice" style=""><tr>
<td class="mbox-empty-cell"></td>
<td class="mbox-text" style="">This section is incomplete<br>Reason: mini-example </td>
</tr></table>
</div>
<div class="t-li1">
<span class="t-li">4)</span> when a <a href="friend.html%23Template_friends" title="cpp/language/friend">friend function declaration</a>, an <a href="function_template.html%23Explicit_instantiation" title="cpp/language/function template">explicit instantiation</a>, or an <a href="template_specialization.html" title="cpp/language/template specialization">explicit specialization</a> refers to a function template specialization
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> X<span class="sy1">&gt;</span> <span class="kw4">void</span> f<span class="br0">(</span>X a<span class="br0">)</span><span class="sy4">;</span>  <span class="co1">// first template f</span>
<span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> X<span class="sy1">&gt;</span> <span class="kw4">void</span> f<span class="br0">(</span>X<span class="sy2">*</span> a<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// second template f</span>
<span class="kw1">template</span><span class="sy1">&lt;&gt;</span> <span class="kw4">void</span> f<span class="sy1">&lt;&gt;</span><span class="br0">(</span><span class="kw4">int</span> <span class="sy2">*</span>a<span class="br0">)</span> <span class="br0">{</span><span class="br0">}</span> <span class="co1">// explicit specialization</span>
 <span class="co1">// template argument deduction comes up with two candidates:</span>
 <span class="co1">// foo&lt;int*&gt;(int*) and f&lt;int&gt;(int*)</span>
 <span class="co1">// partial ordering selects f&lt;int&gt;(int*) as more specialized</span></pre></div></div>
</div>
<p>To determine which of any two function templates is more specialized, the partial ordering process first transforms one of the two templates as follows:
</p>
<ul>
<li> For each type, non-type, and template parameter, including parameter packs, a unique fictitious type, value, or template is generated and substituted into function type of the template
</li>
<li> If only one of the two function templates being compared is a non-static member of some class <code>A</code>, a new first parameter is inserted into its parameter list, whose type is <code>cv A&amp;&amp;</code> if the member function template is &amp;&amp;-qualified and <code>cv A&amp;</code> otherwise (cv is the cv-qualification of the member function template) -- this helps the ordering of operators, which are looked up both as member and as non-member functions:
</li>
</ul>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">struct</span> A <span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span>
<span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> T<span class="sy1">&gt;</span> <span class="kw1">struct</span> B <span class="br0">{</span>
  <span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> R<span class="sy1">&gt;</span> <span class="kw4">int</span> operator<span class="sy2">*</span><span class="br0">(</span>R<span class="sy3">&amp;</span><span class="br0">)</span><span class="sy4">;</span>                     <span class="co1">// #1</span>
<span class="br0">}</span><span class="sy4">;</span>
<span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> T, <span class="kw1">class</span> R<span class="sy1">&gt;</span> <span class="kw4">int</span> operator<span class="sy2">*</span><span class="br0">(</span>T<span class="sy3">&amp;</span>, R<span class="sy3">&amp;</span><span class="br0">)</span><span class="sy4">;</span>          <span class="co1">// #2</span>
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span>
  A a<span class="sy4">;</span>
  B<span class="sy1">&lt;</span>A<span class="sy1">&gt;</span> b<span class="sy4">;</span>
  b <span class="sy2">*</span> a<span class="sy4">;</span> <span class="co1">// template argument deduction for int B&lt;A&gt;::operator*(R&amp;) gives R=A </span>
         <span class="co1">//                             for int operator*(T&amp;, R&amp;), T=B&lt;A&gt;, R=A</span>
<span class="co1">// For the purpose of partial ordering, the member template B&lt;A&gt;::operator*</span>
<span class="co1">// is transformed into template&lt;class R&gt; int operator*(B&lt;A&gt;&amp;, R&amp;);</span>
<span class="co1">// partial ordering between </span>
<span class="co1">//     int operator*(   T&amp;, R&amp;)  T=B&lt;A&gt;, R=A</span>
<span class="co1">// and int operator*(B&lt;A&gt;&amp;, R&amp;)  R=A </span>
<span class="co1">// selects int operator*(B&lt;A&gt;&amp;, A&amp;) as more specialized</span></pre></div></div>
<p>After one of the two templates was transformed as described above, <a href="template_argument_deduction.html" title="cpp/language/template argument deduction">template argument deduction</a> is executed using the transformed template as the argument template and the original template type of the other template as the parameter template. The process is then repeated using the second template (after transformations) as the argument and the first template in its original form as the parameter.
</p>
<p>The types used to determine the order depend on the context:
</p>
<ul>
<li> in the context of a function call, the types are those function parameter types for which the function call has arguments (default function arguments, parameter packs, and ellipsis parameters are not considered -- see examples below)
</li>
<li> in the context of a call to a user-defined conversion function, the return types of the conversion function templates are used
</li>
<li> in other contexts, the function template type is used
</li>
</ul>
<p>Each type from the list above from the parameter template is deduced. Before deduction begins, each parameter <code><b>P</b></code> of the parameter template and the corresponding argument <code><b>A</b></code> of the argument template is adjusted as follows:
</p>
<ul>
<li> If both <code><b>P</b></code> and <code><b>A</b></code> are reference types before, determine which is more cv-qualified (in all other cases, cv-qualificiations are ignored for partial ordering purposes)
</li>
<li> If <code><b>P</b></code> is a reference type, it is replaced by the type referred to
</li>
<li> If <code><b>A</b></code> is a reference type, it is replaced by the type referred to
</li>
<li> If <code><b>P</b></code> is cv-qualified, <code><b>P</b></code> is replaced with cv-unqualified version of itself
</li>
<li> If <code><b>A</b></code> is cv-qualified, <code><b>A</b></code> is replaced with cv-unqualified version of itself
</li>
<li> If <code><b>A</b></code> was transformed from a function parameter pack and <code>P</code> is not a parameter pack, deduction fails
</li>
</ul>
<p>After these adjustments, deduction of <code><b>P</b></code> from <code><b>A</b></code> is done following <a href="template_argument_deduction.html%23Deduction_from_a_type" title="cpp/language/template argument deduction">template argument deduction from a type</a>.
</p>
<p>If the argument <code><b>A</b></code> of the transformed template-1 can be used to deduce the corresponding parameter <code><b>P</b></code> of template-2, but not vice versa, then this <code><b>A</b></code> is more specialized than <code><b>P</b></code> with regards to the type(s) that are deduced by this P/A pair.
</p>
<p>If deduction succeeds in both directions, and the original <code><b>P</b></code> and <code><b>A</b></code> were reference types, then additional tests are made:
</p>
<ul>
<li> If <code><b>A</b></code> was lvalue reference and <code><b>P</b></code> was rvalue reference, A is considered to be more specialized than P
</li>
<li> If <code><b>A</b></code> was more cv-qualified than <code><b>P</b></code>, A is considered to be more specialized than P
</li>
</ul>
<p>In all other cases, neither template is more specialized than the other with regards to the type(s) deduced by this P/A pair.
</p>
<p>After considering every P and A in both directions, if, for each type that was considered,
</p>
<ul>
<li> template-1 is at least as specialized as template-2 for all types
</li>
<li> template-1 is more specialized than template-2 for some types
</li>
<li> template-2 is not more specialized than template-1 for any types OR is not at least as specialized for any types
</li>
</ul>
<p>Then template-1 is more specialized than template-2. If the conditions above are true after switching template order, than template-2 is more specialized than template-1. Otherwise, neither template is more specialized than the other.
</p>
<p>If, after considering all pairs of overloaded templates, there is one that is unambiguously more specialized than all others, that template's specialization is selected, otherwise compilation fails.
</p>
<p>In the following examples, the fictitious arguments will be called U1, U2
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> T<span class="sy1">&gt;</span> <span class="kw4">void</span> f<span class="br0">(</span>T<span class="br0">)</span><span class="sy4">;</span>        <span class="co1">// template #1</span>
<span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> T<span class="sy1">&gt;</span> <span class="kw4">void</span> f<span class="br0">(</span>T<span class="sy2">*</span><span class="br0">)</span><span class="sy4">;</span>       <span class="co1">// template #2</span>
<span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> T<span class="sy1">&gt;</span> <span class="kw4">void</span> f<span class="br0">(</span><span class="kw4">const</span> T<span class="sy2">*</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// template #3</span>
<span class="kw4">void</span> m<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span>
  <span class="kw4">const</span> <span class="kw4">int</span><span class="sy2">*</span> p<span class="sy4">;</span>
  f<span class="br0">(</span>p<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// overload resolution picks: #1: void f(T ) [T = const int *]</span>
        <span class="co1">//                            #2: void f(T*) [T = const int]</span>
        <span class="co1">//                            #3: void f(const T *) [T = int]</span>
<span class="co1">// partial ordering</span>
<span class="co1">// #1 from transformed #2: void(T) from void(U1*): P=T A=U1*: deduction ok: T=U1*</span>
<span class="co1">// #2 from transformed #1: void(T*) from void(U1): P=T* A=U1: deduction fails</span>
<span class="co1">// #2 is more specialized than #1 with regards to T</span>
<span class="co1">// #1 from transformed #3: void(T) from void(const U1*): P=T, A=const U1*: ok</span>
<span class="co1">// #3 from transformed #1: void(const T*) from void(U1): P=const T*, A=U1: fails</span>
<span class="co1">// #3 is more specialized than #1 with regards to T</span>
<span class="co1">// #2 from transformed #3: void(T*) from void(const U1*): P=T* A=const U1*: ok</span>
<span class="co1">// #3 from transformed #2: void(const T*) from void(U1*): P=const T* A=U1*: fails</span>
<span class="co1">// #3 is more specialized than #2 with regards to T</span>
<span class="co1">// result: #3 is selected</span>
<span class="co1">// in other words, f(const T*) is more specialized than f(T) or f(T*)</span>
<span class="br0">}</span></pre></div></div>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> T<span class="sy1">&gt;</span> <span class="kw4">void</span> f<span class="br0">(</span>T, T<span class="sy2">*</span><span class="br0">)</span><span class="sy4">;</span>    <span class="co1">// #1</span>
<span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> T<span class="sy1">&gt;</span> <span class="kw4">void</span> f<span class="br0">(</span>T, <span class="kw4">int</span><span class="sy2">*</span><span class="br0">)</span><span class="sy4">;</span>  <span class="co1">// #2</span>
<span class="kw4">void</span> m<span class="br0">(</span><span class="kw4">int</span><span class="sy2">*</span> p<span class="br0">)</span> <span class="br0">{</span>
    f<span class="br0">(</span><span class="nu0">0</span>, p<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// deduction for #1: void f(T, T*) [T = int]</span>
             <span class="co1">// deduction for #2: void f(T, int*) [T = int]</span>
 <span class="co1">// partial ordering:</span>
 <span class="co1">// #1 from #2: void(T,T*) from void(U1,int*): P1=T, A1=U1: T=U1</span>
 <span class="co1">//                                            P2=T*, A2=int*: T=int: fails</span>
 <span class="co1">// #2 from #1: void(T,int*) from void(U1,U2*) : P1=T A1=U1: T=U1</span>
 <span class="co1">//                                              P2=int* A2=U2*: fails</span>
 <span class="co1">// neither is more specialized w.r.t T, the call is ambiguous</span>
<span class="br0">}</span></pre></div></div>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> T<span class="sy1">&gt;</span> <span class="kw4">void</span> g<span class="br0">(</span>T<span class="br0">)</span><span class="sy4">;</span>  <span class="co1">// template #1</span>
<span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> T<span class="sy1">&gt;</span> <span class="kw4">void</span> g<span class="br0">(</span>T<span class="sy3">&amp;</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// template #2</span>
<span class="kw4">void</span> m<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span>
  <span class="kw4">float</span> x<span class="sy4">;</span>
  g<span class="br0">(</span>x<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// deduction from #1: void g(T ) [T = float]</span>
        <span class="co1">// deduction from #2: void g(T&amp;) [T = float]</span>
<span class="co1">// partial ordering</span>
<span class="co1">// #1 from #2: void(T) from void(U1&amp;): P=T, A=U1 (after adjustment), ok</span>
<span class="co1">// #2 from #1: void(T&amp;) from void(U1): P=T (after adjustment), A=U1: ok</span>
<span class="co1">// neither is more specialized w.r.t T, the call is ambiguous</span>
<span class="br0">}</span></pre></div></div>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> T<span class="sy1">&gt;</span> <span class="kw1">struct</span> A <span class="br0">{</span> A<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="br0">}</span><span class="sy4">;</span>
 
<span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> T<span class="sy1">&gt;</span> <span class="kw4">void</span> h<span class="br0">(</span><span class="kw4">const</span> T<span class="sy3">&amp;</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// #1</span>
<span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> T<span class="sy1">&gt;</span> <span class="kw4">void</span> h<span class="br0">(</span>A<span class="sy1">&lt;</span>T<span class="sy1">&gt;</span><span class="sy3">&amp;</span><span class="br0">)</span><span class="sy4">;</span>    <span class="co1">// #2</span>
<span class="kw4">void</span> m<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span>
  A<span class="sy1">&lt;</span><span class="kw4">int</span><span class="sy1">&gt;</span> z<span class="sy4">;</span>
  h<span class="br0">(</span>z<span class="br0">)</span><span class="sy4">;</span>  <span class="co1">// deduction from #1: void h(const T &amp;) [T = A&lt;int&gt;]</span>
         <span class="co1">// deduction from #2: void h(A&lt;T&gt; &amp;) [T = int]</span>
 <span class="co1">// partial ordering</span>
 <span class="co1">// #1 from #2: void(const T&amp;) from void(A&lt;U1&gt;&amp;): P=T A=A&lt;U1&gt;: ok T=A&lt;U1&gt;</span>
 <span class="co1">// #2 from #1: void(A&lt;T&gt;&amp;) from void(const U1&amp;): P=A&lt;T&gt; A=const U1: fails</span>
 <span class="co1">// #2 is more specialized than #1 w.r.t T</span>
 
  <span class="kw4">const</span> A<span class="sy1">&lt;</span><span class="kw4">int</span><span class="sy1">&gt;</span> z2<span class="sy4">;</span>
  h<span class="br0">(</span>z2<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// deduction from #1: void h(const T&amp;) [T = A&lt;int&gt;]</span>
         <span class="co1">// deduction from #2: void h(A&lt;T&gt;&amp;) [T = int], but substitution fails</span>
 <span class="co1">// only one overload to choose from, partial ordering not tried, #1 is called</span>
<span class="br0">}</span></pre></div></div>
<p>Since in a call context considers only parameters for which there are explicit call arguments, those function parameter packs, ellipsis parameters, and parameters with default arguments, for which there is no explicit call argument, are ignored:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> T<span class="sy1">&gt;</span>  <span class="kw4">void</span>  f<span class="br0">(</span>T<span class="br0">)</span><span class="sy4">;</span>               <span class="co1">// #1</span>
<span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> T<span class="sy1">&gt;</span>  <span class="kw4">void</span>  f<span class="br0">(</span>T<span class="sy2">*</span>, <span class="kw4">int</span><span class="sy1">=</span><span class="nu0">1</span><span class="br0">)</span><span class="sy4">;</span>       <span class="co1">// #2</span>
<span class="kw4">void</span> m<span class="br0">(</span><span class="kw4">int</span><span class="sy2">*</span> ip<span class="br0">)</span> <span class="br0">{</span>
  <span class="kw4">int</span><span class="sy2">*</span> ip<span class="sy4">;</span>
  f<span class="br0">(</span>ip<span class="br0">)</span><span class="sy4">;</span>     <span class="co1">// calls #2 (T* is more specialized than T)</span>
<span class="br0">}</span></pre></div></div>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span>  T<span class="sy1">&gt;</span>  <span class="kw4">void</span>  g<span class="br0">(</span>T<span class="br0">)</span><span class="sy4">;</span>               <span class="co1">// #1</span>
<span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span>  T<span class="sy1">&gt;</span>  <span class="kw4">void</span>  g<span class="br0">(</span>T<span class="sy2">*</span>, ...<span class="br0">)</span><span class="sy4">;</span>         <span class="co1">// #2</span>
<span class="kw4">void</span> m<span class="br0">(</span><span class="kw4">int</span><span class="sy2">*</span> ip<span class="br0">)</span> <span class="br0">{</span>
   g<span class="br0">(</span>ip<span class="br0">)</span><span class="sy4">;</span>     <span class="co1">// calls #2 (T* is more specialized than T)</span>
<span class="br0">}</span></pre></div></div>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> T, <span class="kw1">class</span> U<span class="sy1">&gt;</span> <span class="kw1">struct</span> A <span class="br0">{</span> <span class="br0">}</span><span class="sy4">;</span>
<span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> T, <span class="kw1">class</span> U<span class="sy1">&gt;</span> <span class="kw4">void</span> f<span class="br0">(</span>U, A<span class="sy1">&lt;</span>U,T<span class="sy1">&gt;</span><span class="sy2">*</span> p <span class="sy1">=</span> <span class="nu0">0</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// #1</span>
<span class="kw1">template</span><span class="sy1">&lt;</span>         <span class="kw1">class</span> U<span class="sy1">&gt;</span> <span class="kw4">void</span> f<span class="br0">(</span>U, A<span class="sy1">&lt;</span>U,U<span class="sy1">&gt;</span><span class="sy2">*</span> p <span class="sy1">=</span> <span class="nu0">0</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// #2</span>
<span class="kw4">void</span> h<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span>
  f<span class="sy1">&lt;</span><span class="kw4">int</span><span class="sy1">&gt;</span><span class="br0">(</span><span class="nu0">42</span>, <span class="br0">(</span>A<span class="sy1">&lt;</span><span class="kw4">int</span>, <span class="kw4">int</span><span class="sy1">&gt;</span><span class="sy2">*</span><span class="br0">)</span><span class="nu0">0</span><span class="br0">)</span><span class="sy4">;</span>  <span class="co1">// calls #2</span>
  f<span class="sy1">&lt;</span><span class="kw4">int</span><span class="sy1">&gt;</span><span class="br0">(</span><span class="nu0">42</span><span class="br0">)</span><span class="sy4">;</span>                   <span class="co1">// error: ambiguous</span>
<span class="br0">}</span></pre></div></div>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> T           <span class="sy1">&gt;</span>  <span class="kw4">void</span> g<span class="br0">(</span>T, T <span class="sy1">=</span> T<span class="br0">(</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>  <span class="co1">// #1</span>
<span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> T, <span class="kw1">class</span>... <span class="me1">U</span><span class="sy1">&gt;</span> <span class="kw4">void</span> g<span class="br0">(</span>T, U ...<span class="br0">)</span><span class="sy4">;</span>    <span class="co1">// #2</span>
<span class="kw4">void</span> h<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span>
  g<span class="br0">(</span><span class="nu0">42</span><span class="br0">)</span><span class="sy4">;</span>  <span class="co1">// error: ambiguous</span>
<span class="br0">}</span></pre></div></div>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span>  T, <span class="kw1">class</span>... <span class="me1">U</span><span class="sy1">&gt;</span> <span class="kw4">void</span> f<span class="br0">(</span>T, U...<span class="br0">)</span><span class="sy4">;</span>           <span class="co1">// #1</span>
<span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span>  T            <span class="sy1">&gt;</span> <span class="kw4">void</span> f<span class="br0">(</span>T<span class="br0">)</span><span class="sy4">;</span>                 <span class="co1">// #2</span>
<span class="kw4">void</span> h<span class="br0">(</span><span class="kw4">int</span> i<span class="br0">)</span> <span class="br0">{</span>
  f<span class="br0">(</span><span class="sy3">&amp;</span>i<span class="br0">)</span><span class="sy4">;</span>        <span class="co1">// error: ambiguous</span>
<span class="br0">}</span></pre></div></div>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span>  T, <span class="kw1">class</span>... <span class="me1">U</span><span class="sy1">&gt;</span> <span class="kw4">void</span> g<span class="br0">(</span>T<span class="sy2">*</span>, U...<span class="br0">)</span><span class="sy4">;</span>          <span class="co1">// #1</span>
<span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span>  T            <span class="sy1">&gt;</span> <span class="kw4">void</span> g<span class="br0">(</span>T<span class="br0">)</span><span class="sy4">;</span>                 <span class="co1">// #2</span>
<span class="kw4">void</span> h<span class="br0">(</span><span class="kw4">int</span> i<span class="br0">)</span> <span class="br0">{</span>
  g<span class="br0">(</span><span class="sy3">&amp;</span>i<span class="br0">)</span><span class="sy4">;</span>        <span class="co1">// OK: calls #1 (T* is more specialized than T)</span>
<span class="br0">}</span></pre></div></div>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span>... <span class="me1">Args</span><span class="sy1">&gt;</span>           <span class="kw4">void</span> f<span class="br0">(</span>Args... <span class="me1">args</span><span class="br0">)</span><span class="sy4">;</span>               <span class="co1">// #1</span>
<span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> T1, <span class="kw1">class</span>... <span class="me1">Args</span><span class="sy1">&gt;</span> <span class="kw4">void</span> f<span class="br0">(</span>T1 a1, Args... <span class="me1">args</span><span class="br0">)</span><span class="sy4">;</span>        <span class="co1">// #2</span>
<span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> T1, <span class="kw1">class</span> T2<span class="sy1">&gt;</span>      <span class="kw4">void</span> f<span class="br0">(</span>T1 a1, T2 a2<span class="br0">)</span><span class="sy4">;</span>               <span class="co1">// #3</span>
f<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>                  <span class="co1">// calls #1</span>
f<span class="br0">(</span><span class="nu0">1</span>, <span class="nu0">2</span>, <span class="nu0">3</span><span class="br0">)</span><span class="sy4">;</span>           <span class="co1">// calls #2</span>
f<span class="br0">(</span><span class="nu0">1</span>, <span class="nu0">2</span><span class="br0">)</span><span class="sy4">;</span>              <span class="co1">// calls #3; non-variadic template #3 is more</span>
                      <span class="co1">// specialized than the variadic templates #1 and #2</span></pre></div></div>
<p>During template argument deduction within the partial ordering process, template parameters don't require to be matched with arguments, if the argument is not used in any of the types considered for partial ordering
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">template</span> <span class="sy1">&lt;</span><span class="kw1">class</span> T<span class="sy1">&gt;</span>          T f<span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span><span class="sy4">;</span>  <span class="co1">// #1</span>
<span class="kw1">template</span> <span class="sy1">&lt;</span><span class="kw1">class</span> T, <span class="kw1">class</span> U<span class="sy1">&gt;</span> T f<span class="br0">(</span>U<span class="br0">)</span><span class="sy4">;</span>    <span class="co1">// #2</span>
<span class="kw4">void</span> g<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span>
  f<span class="sy1">&lt;</span><span class="kw4">int</span><span class="sy1">&gt;</span><span class="br0">(</span><span class="nu0">1</span><span class="br0">)</span><span class="sy4">;</span>  <span class="co1">// specialization of #1 is explicit: T f(int) [T = int]</span>
              <span class="co1">// specialization of #2 is deduced:  T f(U) [T = int, U = int]</span>
<span class="co1">// partial ordering (only considering the argument type)</span>
<span class="co1">// #1 from #2: T(int) from U1(U2): fails</span>
<span class="co1">// #2 from #1: T(U) from U1(int): ok: U=int, T unused</span>
<span class="co1">// calls #1</span>
<span class="br0">}</span></pre></div></div>
<p>Partial ordering of function templates containing template parameter packs is independent of the number of deduced arguments for those template parameter packs.
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span>...<span class="sy1">&gt;</span> <span class="kw1">struct</span> Tuple <span class="br0">{</span> <span class="br0">}</span><span class="sy4">;</span>
<span class="kw1">template</span><span class="sy1">&lt;</span>          <span class="kw1">class</span>... <span class="me1">Types</span><span class="sy1">&gt;</span> <span class="kw4">void</span> g<span class="br0">(</span>Tuple<span class="sy1">&lt;</span>Types ...<span class="sy1">&gt;</span><span class="br0">)</span><span class="sy4">;</span>        <span class="co1">// #1</span>
<span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> T1, <span class="kw1">class</span>... <span class="me1">Types</span><span class="sy1">&gt;</span> <span class="kw4">void</span> g<span class="br0">(</span>Tuple<span class="sy1">&lt;</span>T1, Types ...<span class="sy1">&gt;</span><span class="br0">)</span><span class="sy4">;</span>    <span class="co1">// #2</span>
<span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> T1, <span class="kw1">class</span>... <span class="me1">Types</span><span class="sy1">&gt;</span> <span class="kw4">void</span> g<span class="br0">(</span>Tuple<span class="sy1">&lt;</span>T1, Types<span class="sy3">&amp;</span> ...<span class="sy1">&gt;</span><span class="br0">)</span><span class="sy4">;</span>   <span class="co1">// #3</span>
 
g<span class="br0">(</span>Tuple<span class="sy1">&lt;&gt;</span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>                     <span class="co1">// calls #1</span>
g<span class="br0">(</span>Tuple<span class="sy1">&lt;</span><span class="kw4">int</span>, <span class="kw4">float</span><span class="sy1">&gt;</span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>           <span class="co1">// calls #2</span>
g<span class="br0">(</span>Tuple<span class="sy1">&lt;</span><span class="kw4">int</span>, <span class="kw4">float</span><span class="sy3">&amp;</span><span class="sy1">&gt;</span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>          <span class="co1">// calls #3</span>
g<span class="br0">(</span>Tuple<span class="sy1">&lt;</span><span class="kw4">int</span><span class="sy1">&gt;</span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>                  <span class="co1">// calls #3</span></pre></div></div>
<p><br>
</p>
<table class="metadata plainlinks ambox mbox-small-left ambox-notice" style=""><tr>
<td class="mbox-empty-cell"></td>
<td class="mbox-text" style="">This section is incomplete<br>Reason: 14.8.3[temp.over] </td>
</tr></table>
<p>To compile a call to a function template, the compiler has to decide between non-template overloads, template overloads, and the specializations of the template overloads.
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">template</span><span class="sy1">&lt;</span> <span class="kw1">class</span> T <span class="sy1">&gt;</span> <span class="kw4">void</span> f<span class="br0">(</span>T<span class="br0">)</span><span class="sy4">;</span>              <span class="co1">// #1: template overload</span>
<span class="kw1">template</span><span class="sy1">&lt;</span> <span class="kw1">class</span> T <span class="sy1">&gt;</span> <span class="kw4">void</span> f<span class="br0">(</span>T<span class="sy2">*</span><span class="br0">)</span><span class="sy4">;</span>             <span class="co1">// #2: template overload</span>
<span class="kw4">void</span>                     f<span class="br0">(</span><span class="kw4">double</span><span class="br0">)</span><span class="sy4">;</span>         <span class="co1">// #3: nontemplate overload</span>
<span class="kw1">template</span><span class="sy1">&lt;&gt;</span>          <span class="kw4">void</span> f<span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span><span class="sy4">;</span>            <span class="co1">// #4: specialization of #1</span>
 
f<span class="br0">(</span><span class="st0">'a'</span><span class="br0">)</span><span class="sy4">;</span>        <span class="co1">// calls #1</span>
f<span class="br0">(</span>new <span class="kw4">int</span><span class="br0">(</span><span class="nu0">1</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// calls #2</span>
f<span class="br0">(</span><span class="nu16">1.0</span><span class="br0">)</span><span class="sy4">;</span>        <span class="co1">// calls #3</span>
f<span class="br0">(</span><span class="nu0">1</span><span class="br0">)</span><span class="sy4">;</span>          <span class="co1">// calls #4</span></pre></div></div>
<p>Note that only non-template and primary template overloads participate in overload resolution. The specializations are not overloads and are not considered. Only after the overload resolution selects the best-matching primary function template, its specializations are examined to see if one is a better match.
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">template</span><span class="sy1">&lt;</span> <span class="kw1">class</span> T <span class="sy1">&gt;</span> <span class="kw4">void</span> f<span class="br0">(</span>T<span class="br0">)</span><span class="sy4">;</span>    <span class="co1">// #1: overload for all types</span>
<span class="kw1">template</span><span class="sy1">&lt;&gt;</span>          <span class="kw4">void</span> f<span class="br0">(</span><span class="kw4">int</span><span class="sy2">*</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// #2: specialization of #1 for pointers to int</span>
<span class="kw1">template</span><span class="sy1">&lt;</span> <span class="kw1">class</span> T <span class="sy1">&gt;</span> <span class="kw4">void</span> f<span class="br0">(</span>T<span class="sy2">*</span><span class="br0">)</span><span class="sy4">;</span>   <span class="co1">// #3: overload for all pointer types</span>
 
f<span class="br0">(</span>new <span class="kw4">int</span><span class="br0">(</span><span class="nu0">1</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// calls #3, even though specialization of #1 would be a perfect match</span></pre></div></div>
<p>For detailed rules on overload resolution, see <a href="overload_resolution.html" title="cpp/language/overload resolution">overload resolution</a>
</p>
<h3>
<span class="editsection">[<a href="http%3A//en.cppreference.com/mwiki/index.php" title="Edit section: Function template specialization">edit</a>]</span> <span class="mw-headline" id="Function_template_specialization">Function template specialization</span>
</h3>
<table class="metadata plainlinks ambox mbox-small-left ambox-notice" style=""><tr>
<td class="mbox-empty-cell"></td>
<td class="mbox-text" style="">This section is incomplete<br>Reason: 14.8[temp.fct.spec] (note that 14.8.1[temp.arg.explicit] is already in full specialization article: either function specifics go here: lack of partials, interaction with function overloads, or just refer to that </td>
</tr></table>
<h2>
<span class="editsection">[<a href="http%3A//en.cppreference.com/mwiki/index.php" title="Edit section: See Also">edit</a>]</span> <span class="mw-headline" id="See_Also">See Also</span>
</h2>
<ul>
<li> <a href="class_template.html" title="cpp/language/class template">class template</a>
</li>
<li> <a href="function.html" title="cpp/language/function">function declaration</a>
</li>
</ul>

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